`duplicate object description` when redefing variable with `if` block

Especially it is raised when trying generate docs for conf.py in RTD, because RTD modify conf.py.

# conf.py while RTD generating docs
...
html_static_path = ["_static"]
...

###########################################################################
#          auto-created readthedocs.org specific configuration            #
###########################################################################
...
# Add RTD Static Path. Add to the end because it overwrites previous files.
if not 'html_static_path' in globals():
    html_static_path = []
if os.path.exists('_static'):
    html_static_path.append('_static')
...

Sample build from my prod.

Start by reproducing the duplicate object description error while generating documentation for the provided conf.py example in Read the Docs. Trace how the redefined variable inside the if block is processed; done means the documentation build completes without this error.